As oil and gas well completion technology is being extended to greater and greater depths, the need for an improved deep set safety valve increases. Generally, well safety valves are actuated to the open position by a piston and cylinder assembly in response to hydraulic fluid applied to a hydraulic control line extending from the safety valve to the well surface. As the depth at which the safety valve is set increases, the hydrostatic head in the fluid control line to the piston and cylinder assembly increases and must be overcome in order to close the safety valve. In designing safety valves, the tubing pressure as a closing force should not be relied upon since it may not always be there or may be quite variable. Therefore, the safety valve must have another energy source to insure that the safety valve will be fail-safe and will close under all conditions. Another type of closing force has been provided by utilizing a biasing spring action to close the valve. However, the maximum spring force that can be provided is limited and therefore is not sufficient in deep set wells to overcome the hydrostatic opposing force in the control line. Another proposed solution to compensating for the hydrostatic head in the control line is to provide a second or balance line which is connected to the second side of the piston and cylinder assembly and which extends to the well surface to provide a balancing hydrostatic head on the piston and cylinder assembly. Theoretically, this solution would be successful if the various seals in the safety valve never fail. However, in the event of a seal failure, gas in the well fluid would migrate into the balancing line, dilute the density of the balancing hydrostatic head resulting in unbalancing of the hydrostatic head forces thereby preventing the valve from closing.
The present invention is directed to a fail-safe safety valve and method of operation to insure that the hydrostatic head in the hydraulic control line is balanced, and preferably is overbalanced to provide a fail-safe closing force while at the same time the piston and cylinder assembly is protected from the balancing fluid, and the balancing fluid is not subjected to having its density diluted to an extent that it is unable to provide a fail-safe closing force.